This application proposes to explore a novel signaling pathway regulating the process of pathologic cardiac remodeling. Cardiac remodeling represents the pathologic alterations in ventricular structure and function which contribute to heart failure and death. Our laboratory has explored downstream anti-remodeling signaling regulated by the signaling molecule protein kinase G I? (PKGI?). Our prior published studies have revealed a direct role of PKGI? in inhibiting pathologic cardiac remodeling in vivo. PKGI activating therapies are under investigation in humans with heart failure, but have been limited primarily by excess hypotension arising from PKGI induced vasodilation. In preliminary studies we have therefore explored downstream PKGI? substrates in the myocardium in order to identify signaling mechanism which may specifically inhibit remodeling but avoid excess vasodilation. Our preliminary data reveal mixed linage kinase 3 (MLK3) as a PKGI? anti-remodeling substrate. They also identify increased LV pressure overload-induced cardiac remodeling in MLK3 knockout mice, as well as systemic hypertension in these mice. Based on these and other preliminary data we propose to test the hypothesis that MLK3 mediates PKGI? anti-remodeling effects through kinase-dependent effects in the CM, and kinase-independent effects on vascular tone. We propose to test this hypothesis through 3 specific aims: SA1) To determine the role of MLK3 in mediating the anti-remodeling effects of PKGI? signaling; SA2) To determine the kinase-dependent mechanisms through which MLK3 inhibits cardiac remodeling; SA3) To determine the kinase-independent mechanisms through which MLK3 reduces blood pressure. These proposed studies utilize in vitro approaches as well as multiple innovative mouse models. Successful completion of these studies has the potential to identify MLK3 kinase activation as a novel therapeutic strategy for cardiac remodeling. Further, these studies will provide experimental evidence to address the novel paradigm that PKGI? inhibits cardiac remodeling and reduces blood pressure through independent signaling mechanisms.